This invention relates to a method of catalytic combustion using a heat-resistant catalyst. More particularly, the present invention is concerned with a method of catalytic combustion using a heat-resistant catalyst suited for combustion at a temperature ranging from 800.degree. to 1500.degree. C.
Known catalysts which have been employed as a high temperature catalyst include a catalyst comprising a carrier such as alumina, silica or silica-alumina and a noble metal or base metal carried thereon, and a catalyst composed of a carrier comprising a ceramic material such as zirconia, aluminum titanate, cordierite or silicon nitride having a coating of an active alumina or the like on its surface and a noble metal component carried on the coating. However, these catalysts have a drawback that when the temperature is as high as 800.degree. C. or above, they generally experience alteration of a crystalline structure of the carrier (e.g., phase transition from .gamma.-type to .alpha.-type in the case of alumina) and/or the specific surface area decrease accompanying crystal growth, thereby causing a decrease in active sites due to agglomeration of active components, which in turn leads to a lowered catalytic activity. For example, a conventional catalyst composed of a carrier comprising alumina and palladium carried thereon has a specific surface area of about 150 m.sup.2 /g and a palladium particle diameter of 30 .ANG. after heat treatment at 600.degree. C., but it comes to have a specific surface area of about 3 m.sup.2 /g and a palladium particle diameter of about 200 .ANG. upon being heated at 1200.degree. C. for 2 hr. That is, due to heat treatment at a high temperature, palladium particles which are finely dispersed on the carrier agglomerate into large particles as a result of a decrease in the specific surface area of the carrier, thereby causing the active sites of the catalyst to decrease, so that a decrease in catalytic activity is brought about.
As an improved catalyst over the above-mentioned catalysts for high temperatures, Japanese Patent Laid-Open No. 52529/1984 proposes a catalyst which suppresses agglomeration of a noble metal through the use of a carrier prepared by coating a heat-resistant carrier with a mixture of .gamma.-alumina with cerium, lanthanum, strontium, tin, zirconium or magnesium and a ceramic whisker. On the other hand, Japanese Patent Laid-Open No. 169536/1984 proposes a catalyst prepared by coating a heat-resistant carrier with a mixture of .gamma.-alumina with langthanum or cerium and strontium, contacting particles of a base metal such as nickel, chromium or cobalt with the coated carrier, and further depositing platinum or palladium thereon. Although each of these improved catalysts has its own advantages, the heat resistance is not satisfactory.
Meanwhile, although its application field is different from that intended in the present invention, a catalyst for the oxidative cleaning of exhaust gases of automobiles is disclosed in a Japanese Patent Laid-Open No. 26390/1977. This catalyst has active sites of platinum or palladium decreased by addition of a metal of different kind such as magnesium or barium, and is intended to be used at a temperature ranging from 300.degree. C. to 600.degree. C.
As disclosed in Japanese Patent Laid-Open No. 222145/1985, it was found that an effective catalyst could be obtained by the use of a carrier comprising lanthanum-.beta.-alumina (La.sub.2 O.sub.3 .multidot.11.about.14 Al.sub.2 O.sub.3) produced by adding lanthanum to aluminum, which was improved in thermal stability as compared with the conventional carriers so as to prevent the active component particles carried thereon from undergoing agglomeration, in combination with a catalytically active component such as a noble metal or a transition metal. It was confirmed by the nitrogen adsorption test that the lanthanum-.beta.-alumina had a low specific surface area at high temperatures. As a result of an electron microscopic observation of the active component particles on the carrier at high temperatures, it was confirmed that the particles had a small diameter and were finely dispersed. For example, a catalyst composed of lanthanum-.beta.-alumina and palladium carrier thereon, which was calcined at 1200.degree. C., had a specific surface area of about 30 m.sup.2 /g and a palladium (Pd) particle diameter of about 700 .ANG., thus being superior in heat resistance to the forementioned catalyst having a carrier solely comprising alumina. With respect to this catalyst composed of a carrier comprising lanthanum-.beta.-alumina and a noble metal or base metal as a catalytically active component carried thereon, the active component agglomeration due to the specific surface area decrease of the carrier by the use at high temperatures can be desirably obviated. However, this catalyst cannot be free from agglomeration of active component particles at high temperatures, which agglomeration is independent of the changes of the carrier. Therefore, it is necessary to further improve the heat resistance of the catalyst by suppressing the agglomeration of active component particles.